I N THE year 2002, the US Federal Communications Com-

mission (FCC) allocated the ultrawideband (UWB) spec-trum from 3.1 to 10.6 GHz to be used for commercial commu- band-notch elements. Different multiple (dual, triple, quadruple) band-notched UWB antenna topologies havenication purposes [1]. UWB communication systems can sup- also been reported in recent literature [6][9]. Use ofport high data rates with low power consumption and are mainly split-ring resonators (SRRs) and complementary split-ringused for short-range indoor communication systems. Antennas resonators (CSRRs) to design reconfigurable multiplespanning the broad frequency range of 3.110.6 GHz are es- band-notched UWB antennas has been presented in [10][13].sential components of such UWB systems; hence, their design In this letter, we propose a simple low-cost microstrip-fedproblem has become an active topic of research in recent years. half-elliptic monopole antenna similar to [2], with tripleUWB antenna engineers have to simultaneously ensure high band-notched functionality (Fig. 1). The half-elliptic monopoleimpedance bandwidth, radiation-pattern stability, miniaturiza- is more compact compared to its complete circular, rectangular,tion, along with low manufacturing cost in their design works. or elliptic counterparts as used in [10] and [11]. Two ellipticApart from these issues, the problem of electromagnetic inter- single complementary split-ring resonators (ESCSRRs) areference (EMI) occurring due to existing narrowband communi- embedded in the radiating patch to cancel interferences duecation systems like WiMAX (3.33.8 GHz), WLAN (5.155.85 to WiMAX (3.33.8 GHz) and upper WLAN frequenciesGHz), or X-band satellite communication links (7.98.4 GHz) (5.155.85 GHz). It can be inferred from [11] that for genera-is of major concern. Integration of external bandstop filters to tion of narrower and stronger notch-bands, slots/CSRRs withachieve the desired band rejection increases system complexity shape similar to that of the antenna patch are more effective,and size. Hence, in order to keep the antenna footprint unal- as the current distribution is mostly concentrated at the edgetered, designers have resorted to the approach of embedding of the patch. The use of ESCSRRs for providing band-notchedparasitic strips or slots of different shapes in the radiating el- characteristics is not very common, to the best of the knowl-ement or ground plane of the antenna systems [2][5]. edge of the authors. Thus, a comprehensive and simple design Moreover, many UWB applications require more than one methodology of elliptic CSRRs most suitable for this antenna isnotch-band, necessitating the use of mutually noninteracting developed in this letter. Also, an ellipse having same area as that of a circle would have greater perimeter [14], which intuitively implies that use of an elliptic CSRR or SRR would provide Manuscript received December 06, 2013; accepted February 12, 2014. Dateof publication February 19, 2014; date of current version March 06, 2014. lesser resonant frequency. This indicates miniaturization of The authors are with the Electrical Engineering Department, Indian Insti- the band-reject elements. Furthermore, rectangular split-ringtute of Technology, Kanpur 208016, India (e-mail: debdeep1989@gmail.com; resonators are placed as parasitic elements near the junctionkvs@iitk.ac.in; kushmandasaurav@gmail.com). Color versions of one or more of the figures in this letter are available online of feedline and radiating element to reject the ITU-speci-at http://ieeexplore.ieee.org. fied X-band communication frequencies (7.98.4 GHz). The Digital Object Identifier 10.1109/LAWP.2014.2306812 proposed antenna, designed on FR4 substrate, is compact in

desired notch frequency. The factor used for the calcula-

tion of the circumference of ellipse is related to the ellipticity ) through (2) [14]. The effective dielectric con- stant is calculated via (3), where , and are the substrate height, width of the microstrip feed, and relative permittivity of substrate, respectively [15]. In our design simulations, we have fixed the width of ESCSRR slots at 0.3 mm, keeping the limitations of the available fabrication facility in mind. Thus, for a given value of , we have the option of properly choosing and from (1) and (2). If ellipticity is specified, we obtainFig. 2. Photograph of the fabricated antenna: (a) top view; (b) bottom view. from (2) and solve for using (1). However, for a speci- fied value of , we can generate a simple quadratic equation from (1) and (2) and solve for analytically.size (35 35 1.6 mm ), and it exhibits monopolar far-fieldpattern in the radiating band. The details of the antenna design B. Initial Choice of Rectangular SRR Dimensionsprocedure and its radiation performance are described in thefollowing sections. For choosing the design parameters of the rectangular SRR for a desired , we can use the design guidelines from the II. ANTENNA DESIGN METHODOLOGY following equation: The topology of the proposed antenna is shown in (4)Fig. 1. The antenna radiator is a half-elliptic patch havingmajor axis length and minor axislength and placed on one side of the Here, is the inner perimeter of the rectangular single splitsubstrate. Here, and functions respectively ring that is a function of ring length , ring width , andchoose the maximum value and minimum value from the strip width , as shown in Fig. 1(c). For the rectangular SRRarguments. The radiating patch is fed by a 50- microstrip line to resonate at should be approximately equal to halfof width . The ground plane of the antenna is also half-el- of the guided wavelength at that frequency.liptic in shape having major axis lengthand minor axis length . The antenna ground C. Full-Wave Simulationplane is placed symmetrically about the -axis with respect Fig. 3 illustrates the four different stages of the antenna designto the patch, on the other side of the substrate. Fig. 2 shows principle adopted in this letter. The variations of VSWR withthe top and bottom views of the proposed antenna fabricated frequency for these design stages are provided in Fig. 4. Case-Ion 1.6-mm-thick FR4 substrate (dielectric constant 4.4, loss shows the reference UWB antenna. In case-II, one ESCSRRtangent 0.02). [denoted by in Fig. 1(a)] is etched out to obtain a single First, the reference antenna structure is simulated by finite el- band-notched antenna structure. To get this first band-notch atement method (FEM)-based commercial electromagnetic sim- 5.75 GHz within the WLAN band (5.155.85 GHz), we chooseulator HFSS. The values of , and are paramet- mm, mm, and analytically obtain therically tuned to get an impedance bandwidth of required value of ellipticity as 1.64. We take this value of9.50 GHz (2.2111.71 GHz), which covers the FCC specified ellipticity as an initial guess and carry out HFSS simulations. Itfrequency range for UWB systems. Our objective is to obtain the is observed that the notch at 5.75 GHz is obtained for ,desired triple band-notched performance by using the ESCSRRs while , and are kept fixed at the values used in the( and in Fig. 1) and rectangular SRRs on the reference UWB analytical prediction step.antenna. Before proceeding to full-wave simulations, we need Next, we symmetrically place two rectangular SRR elementsto make some initial guesses regarding the dimensions of the of suitable dimensions near the junction of the feedline andband-notch elements. This issue is addressed in Sections II-A the radiating patch (Case-III) as shown in Fig. 3. The separa-and II-B. tion between the SRRs and the microstrip feedline has to be kept small to ensure effective coupling resulting in band rejec-A. Initial Choice of ESCSRR Dimensions tion. Keeping the available printed circuit board (PCB) fabri- For an ESCSRR of major axis length , minor axis length cation limitations in mind, this separation is kept at 0.3 mm. , and width [Fig. 1(b)], the design equations for ob- For obtaining the notch at 8.27 GHz (lying within the bandtaining a band-notch at frequency can be written as of 7.98.4 GHz for X-band communication), we start with pa- rameter values mm, mm and get (1) mm from the design guidelines in (4). The notch frequency at 8.27 GHz is obtained at mm from HFSS (2) simulations. In the final proposed design (Case-IV), an additional band- (3) notch at 3.55 GHz within the WiMAX band (3.33.7 GHz) is provided by using another larger ESCSRR element [denoted by Here, denotes the inner circumference of the elliptic slot in Fig. 1(a)] having the following parameter values:of the single complementary split ring, which should be ap- mm, mm, (analytically predictedproximately equal to half of the guided wavelength at the value of ). Strong concentration of simulated surface398 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 13, 2014

monopole antenna with elliptic patch and ground plane. Case-II: configurationfor UWB antenna with rejection of single band by etching out one ESCSRRfrom the radiating patch. Case-III: dual band-notched antenna by use of oneESCSRR and two rectangular SRRs. Case-IV: proposed triple band-notchedantenna using two ESCSRRs and two rectangular SRRs. Fig. 5. Distribution of surface current vector on the antenna structure for the three notch frequencies (a) 3.55, (b) 5.75, and (c) 8.27 GHz.

Fig. 4. Variation of the VSWR with frequency for the four different cases as Fig. 6. Variation of the VSWR with frequency for proposed antenna.mentioned in Fig. 3.

Fig. 2. Fig. 6 illustrates a comparison between the simulated

current vector is observed in the vicinity of the outer ESCSRR and measured VSWR values of the proposed antenna withfor 3.55 GHz, inner ESCSRR for 5.75 GHz, and the rectangular respect to frequency. The fabricated antenna has an impedanceSRRs for 8.27 GHz (Fig. 5), clearly indicating the respective el- bandwidth from 2.21 to 12.83 GHz with notch-bands in theements responsible for the band-notch characteristics. The final frequency ranges of 2.953.72 GHz (design parameters of the antenna are as follows: mm, at 3.54 GHz), 5.126.07 GHz ( at 5.73 mm, mm, mm, mm, GHz), and 8.048.65 GHz ( at 8.56 GHz). mm, mm, These three notch bands correspond to WiMAX, WLAN, and mm, mm, . Here, sub- X-band communication frequencies, respectively. It is ob-scripts and denote the smaller (Inner) and larger (Outer) served that, for the fabricated antenna, the third notch-band forESCSRRs, respectively. rejecting the high frequency band for X-band communication is shifted by almost 300 MHz as compared to simulation. This III. MEASUREMENT RESULTS AND DISCUSSION can be attributed to the fact that the high frequency bands are The proposed antenna is fabricated on low-cost FR4 substrate more sensitive to fluctuation in relative permittivity of the sub- dielectric constant loss tangent as shown in strate used [11]. Moreover, with respect to the first resonanceSARKAR et al.: COMPACT MICROSTRIP-FED TRIPLE BAND-NOTCHED UWB MONOPOLE ANTENNA 399

three desired bands, confirming the fact that the subwavelength

resonators (ESCSRRs and rectangular SRRs) provide excellent intrinsic filtering without the need of external circuitry. Also, it can be seen that, in the radiating band, the gain variation is al- most the same as that of the reference antenna structure.

IV. CONCLUSION In this letter, a compact triple band-notched UWB monopole antenna is realized by embedding elliptic complementary split rings and rectangular split rings in the antenna structure. The proposed antenna has impedance bandwidth covering the en- tire UWB range (3.110.6 GHz), along with notch-bands in the WiMAX (3.33.8 GHz), WLAN (5.155.85 GHz), and X-band (7.98.4 GHz) frequencies. The design guidelines and relevant equations are described and validated via HFSS simulations. A prototype antenna is fabricated in PCB lab using low-cost FR4 substrate. The VSWR and far-field measurements of the fabri- cated antenna exhibit good match with simulation predictions.